Re: Relation extension scalability

On 12/03/16 01:01, Jim Nasby wrote:
> On 3/11/16 5:14 PM, Petr Jelinek wrote:
>>> I don't really understand this part about concurrent DDL. If there
>>> were concurrent DDL going on, presumably other backends would be
>>> blocked on the relation lock, not the relation extension lock - and it
>>> doesn't seem likely that you'd often have a huge pile-up of inserters
>>> waiting on concurrent DDL. But I guess it could happen.
>>>
>>
>> Yeah I was thinking about the latter part and as I said it's very rare
>> case, but I did see something similar couple of times in the wild. It's
>> not objection against committing this patch though, in fact I think it
>> can be committed as is.
>
> FWIW, this is definitely a real possibility in any shop that has very
> high downtime costs and high transaction rates.
>
> I also think some kind of clamp is a good idea. It's not that uncommon
> to run max_connections significantly higher than 100, so the extension
> could be way larger than 16MB. In those cases this patch could actually
> make things far worse as everyone backs up waiting on the OS to extend
> many MB when all you actually needed were a couple dozen more pages.
>

Well yeah I've seen 10k, but not everything will write to same table,
wanted to stay in realms of something that has realistic chance of
happening.

> BTW, how was *20 arrived at? ISTM that if you have a lot of concurrent
> demand for extension that means you're running lots of small DML
> operations, not really big ones. I'd think that would make *1 more
> appropriate.

The benchmarks I've seen showed you want at least *10 and *20 was better.

--
Petr Jelinek http://www.2ndQuadrant.com/
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